Metal Ablation by ps-Laser-Pulses: A Hybrid Simulation

Boundary condition

We use a pressure-transmitting boundary condition for molecular-dynamics
simulations in the last four monolayers of a copper crystal; and otherwise
lateral periodic boundary conditions. The new boundary condition
- the teminating zone - prevents a pressure wave reflection, this is shown
in the next figure.

t = 0.5 1.0
1.5 2.0
2.8 3.2
4.0 10.0 ps

The compressive pressure wave (red colored) travels with the velocity
of sound through the crystal. After 3.2 ps the wave reaches the terminating
zone, where calculation of the forces is changed. The high pressure is
reduced to the nominal crystal pressure. The local pressures is in units
of the bulk modulus, B=137 GPa.

Atomistic view of part of the laser-irradiated solid, at a time t =
32 ps, immediately after ablation occurred. Cross sections through the
simulation volume (height 6 nm, width 2.1 nm, thickness 1 nm) are shown
at various times after laser irradiation at a depth of 23 nm. Atoms are
colored (a) according to their local pressures, in units of the bulk modulus,
B = 137 GPa, and (b) according to their local temperature, in units of
the melting point of copper, Tm = 1358 K. The local temperature
of an atom is defined as the average kinetic energy of all atoms around
the central atom within a radius of 6.2 A (cutoff of the interaction potential),
in the center of mass system. Analogously, local pressures are defined
as an average over the atomic virials.